1. Field of the Invention
The present invention is related to a semiconductor device having metal gate and fabrication method thereof, and more particularly, to a semiconductor device having metal gate and fabrication method thereof with a good hole-filling performance of gate material.
2. Description of the Prior Art
With a trend toward scaling down the size of the semiconductor device, work function metals are used to replace the conventional polysilicon gate to be the control electrode that is competent to the high dielectric constant (herein after abbreviated as high-k) gate dielectric layer. In a conventional fabrication method of work function metal gate, a polysilicon dummy gate (or called as “replacement gate”) is formed and followed by steps of forming a conventional metal-oxide semiconductor (MOS), such as forming spacer and source/drain. Subsequently, the polysilicon dummy gate is removed to form a gate trench. Thereafter, the gate trench is filled with material layers required for different conductivity types, such as gate insulating layer, work function layer and metal layer. However, the aspect ratio of the gate trench is kept on being raised in the developing requirement, resulted in that the difficulty of filling the gate trench with layers becomes higher and higher. Accordingly, voids are easily formed in the gate trench during hole-filling processes, which seriously affect the electric performance of the transistor. Furthermore, in a prior-art fabrication process, a pull-down process of the inner layers of the gate trench is required for enlarging the opening size of inner surface of the gate trench in order to improve the hole-filling performance. However, the above-mentioned pull-down process further complicates the whole fabrication process. As a result, it is still an important issue for the manufacturer to improve the fabrication yield of the metal gate of semiconductor device by simplified process.